<h3>描述</h3>
<p>Buttongroup组件，自定义选中值，设置valueKey时选中值基于valueKey</p>
<p>valueKey接口</p>
<h3>示例</h3>
<p>1.单选：设置valueKey</p>
<ti-button-group valueKey="english" [items]="items" [(ngModel)]="selected"> </ti-button-group>
<p>选中项：{{selected | json}}</p>
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<p>多选：设置valueKey</p>
<ti-button-group valueKey="english" multiple="true" [items]="items" [(ngModel)]="selecteds"> </ti-button-group>
<p>选中项：{{selecteds | json}}</p>
